1. Field of the Invention
This invention concerns a valve intended to disconnect, in special circumstances, the braking of the front wheels and/or of the trailer, in a farm tractor or in a similar vehicle.
2. Description of the Related Art
In certain vehicles, particularly farm tractors and similar vehicles, the hydraulic braking system includes two master cylinders controlled by two separate braking pedals, and each master cylinder controls the braking of one of the rear wheels of the vehicle. This arrangement aims to allow a differential braking of the rear wheels, for example in order to facilitate the vehicle rotation for the U-turn during the cultivation of a field along adjacent lanes which are run in alternatively opposite directions. When a bilateral braking is effected by operating both the braking pedals, in general the system would not be balanced because the two separate braking circuits take up different oil quantities, and also the braking would then not be balanced. In order to prevent this disadvantage, a transfer channel is arranged for mutually communicating the two braking systems when both braking pedals are simultaneously operated. This balancing operation is effected by valves inserted on the pistons of the master cylinders, which valves are normally closed and are opened when the pistons run along a preestablished stroke.
The vehicles equipped with the above system may be devoid of brake devices acting on the front wheels and of brake devices in the trailer possibly dragged. However, some systems are provided with brake devices also acting on the front wheels and/or with brake devices in the trailer, operated through a valve named “trailer brake valve”, and in this case the systems comprise a hydraulic disconnecting valve intended to disconnect the braking of the front wheels and/or of the trailer in those cases in which only one braking pedal is operated, whereas, when both braking pedals are simultaneously operated, all the four wheels of the vehicle and/or the trailer wheels are braked. The block diagram of such a braking system is represented in FIG. 1, and the corresponding hydraulic diagram is represented in FIG. 2. This system will be described with more details later on.
A disadvantage of this kind of braking systems is that the hydraulic disconnection valve has a complicated structure, to which follow high manufacturing and assembling costs as well as relatively easy breakdowns. A typical example of the structure of this type of hydraulic disconnection valve is represented in FIG. 3, and its hydraulic diagram is represented in FIG. 4. This valve is described with more details later on.
Another drawback of the known systems is that the operations for exhausting the air from the hydraulic circuit when filling the same with oil involve rather complicated steps.